Believe it or not, this could be the result of a selection
you don't make in the screen
after this one. Click the Next > button and see what your
setting is for the "Navigation Pane Preferred Format". If you have
it set for "Pure HTML", this could explain it. So set it for one of
the other options and click the < Back button to see if it's now
enabled.

If that didn't work, sometimes things like this happen with
no obvious reason. One thing you might try is to right-click the
Single Source Layouts pseudo folder and declare a brand new layout.
Then see if anything changes.